This invention relates to FM/CW radar, i.e., a radar in which a signal in the form of a periodically frequency modulated carrier is radiated and is, directly or indirectly, compared with an echo signal to obtain a signal representing a beat frequency between the radiated and echo signals. The beat frequency is proportional to the range of the target.
Whether sinusoidal or some other form of periodic modulation of the radiated frequency is used, the beat frequency at times associated with maxima or minima of the transmitted frequency is low either because the rate of change of the transmitted frequency is low at the times of such maxima or minima or because at times near such maxima the difference between the instantaneous frequencies of the radiated signal and the echo signal passes through zero. In consequence, the beat frequency at such times is abnormal; there is in effect a loss of the beat frequency signal during short intervals at times near the peaks of the modulating waveform. The loss of information is not serious, because the duration of the intervals during which an adequate beat frequency is normally produced is usually comparatively so long that an average of the beat frequency signal indicates range sufficiently accurately.